What is cumulative incidence and how is it calculated?

• A. Cumulative incidence is the proportion of all participants who ever develop the disease, regardless of baseline disease status.
• B. Cumulative incidence is the proportion of initially disease-free individuals who develop the outcome during the follow-up; calculated as (new cases during follow-up) / (persons at risk at start).
• C. Cumulative incidence equals the total number of cases divided by total person-time.
• D. Cumulative incidence is the same as the incidence rate.

Answer: (B)

Cumulative incidence measures the probability that a person who is free of the disease at the start of a defined period will develop the disease by the end of that period. It is calculated as the number of new cases that occur during the follow-up divided by the number of people at risk at the beginning (those without the disease at baseline). This focuses on the risk among initially disease-free individuals over a specified time.

Why this matters: it gives the incidence proportion over a set time, not just a count of cases or a rate per person-time. It differs from the incidence rate, which uses person-time in the denominator. If there’s censoring or loss to follow-up, more advanced methods like survival analysis may be needed, but the basic idea remains the proportion of at-risk individuals who develop the disease during the follow-up.